The present invention relates to a light source device for photographic enlargers, and more particularly to a light source unit assembly comprising two types of optical systems one of which is alternatively positionable on the optical axis between a light source and the negative for giving prints of increased or reduced contrast.
Photographic enlargers include an optical element disposed on the optical axis between the light source and the negative for diffusing the rays from the light source and illuminating the negative. Enlarger illumination systems are divided into three general types: diffused illumination, semi-diffused illumination and converged or direct illumination.
With the diffused illumination system, the negative is illuminated with rays diffused in all directions, and only some of the rays fall on the enlarger lens and act effectively for printing. Since the negative is illuminated in all directions with the other rays which are wasted, this system affords prints of soft contrast. Accordingly slight scratches on the negative will not be reproduced in the print. The system is suitable chiefly for enlarging color negatives.
On the other hand, the semi-diffused (or converged-diffused) illumination system is basically adapted to concentrate negative illuminating rays on the enlarger lens with a condenser. The system employs a large diffused light source, such as an opal bulb, is unaffected by variations in the focal distance of the enlarger lens and affords contrasty prints.
Thus the semi-diffused illumination system is best suited to black-and-white photographs which require sharp contrast between black and white. However this system, although usable for color negatives, has the drawback that scratches on the negative become more obvious in the print.
The enlarger light source element for color photography therefore basically differs from the light source element for black-and-white photography, whereas many of amateurs in photography usually use an enlarger of the semi-diffused illumination type or the diffused illumination type only chiefly for economical reasons and are unable to achieve the desired result that otherwise would be fully attained.